Frequently, the dynamic response range of an x-ray imaging system is less than the x-ray attenuation range of the object to be imaged. This situation is encountered often by the conventional x-ray radiography system, which is comprised of a film-screen cassette as the detector-recorder for x-ray images and an x-ray source with a broad and spatially uniform beam. This system has been in popular use since the discovery of x-rays about ninety years ago. The medical x-ray film typically has a very high contrast enhancement factor, which is often called the contrast gradient or gamma, resulting in a very narrow latitude (or exposure range). This high contrast enhancement factor is a necessary feature of an x-ray film because typical anatomical objects to be detected have very low x-ray contrast so that their image on the x-ray film would be too faint for the physicians to see if the contrast enhancement factor were lowered.
In most x-ray examinations, such as a chest examination, the typical patient exhibits very large anatomical thickness variations and, thus, very large x-ray attenuation variations. That is, some anatomical parts are very opaque to x-rays and some other parts are very transparent to x-rays. As a result, x-ray films of these examinations, due to the narrow exposure range of the x-ray film, are optimally exposed for only a portion of the entire picture, leaving large portions of the picture either overexposed or underexposed. The contrast enhancement factor for both the overexposed and the underexposed regions is much lower than that of the optimally exposed regions. Therefore, there is significant loss of x-ray information (and loss of diagnostic value) in the overexposed and underexposed regions of the x-ray film.
For example, in a typical PA chest film, the lung field is usually optimally exposed by choice, and the mediastinal and subdiaphragmatic areas are left underexposed. The probability of detecting tumors and other abnormalities located in the underexposed areas is significantly lower than the detection probability in the lung field, where the exposure is optimum. This non-uniform exposure of the x-ray film, which is due primarily to the large thickness variations in a typical patient, is a major shortcoming in conventional x-ray radiography systems. The image quality and diagnostic value of the x-ray film can be improved significantly if the non-uniform exposure effect caused by these thickness variations in the patient can be reduced.
Indeed, Pennington et al (Proc. SPIE, volume 233, pages 176-182 (1980)), Plewes et al ((A) Radiology, volume 142, pages 765-768 (1982); and (b) Diagnostic Imaging, October 1985, pages 85-96), and others have demonstrated that nodule detection in chest films can be significantly improved with some compensating means by which the non-uniformity in the exposure is reduced. These compensating means all involve spatial modulation of the x-ray flux so that the flux at the x-ray film is more or less equalized. The process is frequently called the flux equalization method.
In order to provide flux equalization to all varieties of patients, feedback controls have to be added to the flux equalization processes. That is, flux equalization is provided after some spatial attenuation information has been obtained on the specific patient who is being examined. It is important to point out here that in carrying out the feedback controlled flux equalization process, one must not generate new problems such as patient motion unsharpness resulting from prolonged exposure time, artifacts from compensation misregistration, increased patient dosage, excessive heat loading on the x-ray tube target, increasing the effect of scattered x-rays, and user or patient inconveniences.
There exists a large body of feedback controlled flux equalization prior art. Representative of the prior art using a x-ray mask are Edholm et al U.S. Pat. No. 3,755,672 and Mistretta et al U.S. Pat. No. 4,497,062. Representative of the prior art using an optical mask is Nelson et al U.S. Pat. No. 4,322,619. Prior art using a raster scanned x-ray target to generate a scanning pencil beam of x-rays is represented by Craig et al U.S. Pat. No. 2,837,657. Prior art using a mechanically moved scanning aperture to generate a scanning pencil beam of x-rays is represented by Plewes et al (Medical Physics, volume 10, pages 655-663 (1983)). Prior art using a scanning fan beam are represented by Fredzell U.S. Pat. No. 4,433,430 and Plewes et al (Radiology, volume 142, pages 765-768 (1982)).
The most pertinent prior art related to the present invention are feedback controlled flux equalization x-ray radiography systems using a scanning fan beam of x-rays. Representative of the prior art are, as mentioned above, Fredzell U.S. Pat. No. 4,433,430 and an article by Plewes et al (Radiology, volume 142, pages 765-768 (1982)). The main advantages of these systems over the systems using the scanning pencil beam of x-rays are: (a) increased x-ray tube life with about 10 to 20 times reduction in heat loading through more efficient use of x-rays, (since heat loading is proportional to the ratio of the area of the imaged field to the area of the scanning aperture and the aperture used in scanning fan beam is usually 10 to 20 times larger than the aperture used in scanning pencil beam,) (b) less patient motion problem through shorter time required to complete the imaging process, (c) less scan artifacts because scanning pencil beam systems require accurate control of the spacing between overlapping scan lines and the size and profile of the x-ray spot, and (d) less cooling time of the x-ray tube associated with reduced heat loading means shorter wait between examinations and higher patient throughput.
The scanning fan beam systems also have many advantages over the systems using x-ray or optical masks. These advantages are: (a) no need of hassle in the making and subsequent registration or alignment of x-ray or optical masks, (b) much less patient misregistration problem resulting from much less time delay between the process of obtaining the patient attenuation information and final imaging process, and (c) much better rejection against scattered x-rays.
However, the main drawback of the fan beam systems taught by Fredzell and Plewes is that the flux equalization is only applied in the direction of the scan and not in the direction perpendicular to the scan. That is, the feedback signal is used to control the x-ray source intensity or duty cycle providing an uniform x-ray intensity across the entire fan beam. This one-dimensional feedback controlled flux equalization is unable to provide compensation to the entire image and is also prone to scan artifacts. Indeed, Plewes et al concluded in the same article that a scanning pencil beam system is the "only" way to overcome this drawback. The system taught by Fredzell has two fan beams. One fan beam is used as the monitoring beam to acquire patient attenuation information, and the second fan beam is used to image. However, since the same x-ray source is being used by both fan beams and the source intensity modulation would also affect the monitoring fan beam, it is not clear how the system's feedback control could function properly. It should also be obvious that Fredzell's systems could not support more than one imaging beam since each imaging beam would require a different modulation.